Just my humble opinion, but, bit more centralized way might look work. For example, each client receives global feed only from paid relays as much as possible.

As you have already done in Amethyst.

The bot and spam runner will lose their benefit if everyone does this so because only just a few people will see the posts from bots if it happens.

Eventually, the number of spams and bots will decrease.

People who want to post globally may need to pay a bit for it, but doesn't sound unfair to me. Now, this cost is paid by people running free relays after all.